The present invention relates to a junction method for joining a plurality of plate materials superimposed in their thickness directions at points and a junction tool used for the junction method.
A resistance welding method and a mechanical fastening method by rivets have been conventionally known as a method for joining a workpiece formed of a plurality of plate materials superimposed in their thickness directions at points.
The resistance welding method has drawbacks in view of running cost and quality control. Namely, the resistance welding method requires a power source with large capacity at the time of junction. A welding electrode has a short lifetime because of contamination or wear. Further, equipment for supplying cooling water is required in order to cool the electrode.
The mechanical fastening method by rivets has problems in that it requires a large number of rivet materials and such rivets lead to an increase in the weight of the resultant joined material.
A friction agitation junction method has been known as a method for joining a workpiece at points (see Japanese Patent Application Laid-Open (JP-A) No. 2001-314982). In accordance with this method, junction is performed for a workpiece by using a junction tool configured by a first tool which includes a pin protruding from its distal end surface and a second tool which has a flat distal end surface.
Specifically, in accordance with the friction agitation junction method, while the first tool is rotated, the workpiece is nipped by the junction tool and pressed. Then, the pin of the first tool is sunk into the workpiece.
Then, the workpiece is softened by a friction heat generated by the rotation of the first tool and the first tool is even further sunk into the workpiece. Further, plastic flow is generated within the workpiece by the rotation of the first tool. The vicinity of the superimposed surface of the workpiece (i.e., surface at which adjacent two plate materials contact) is agitated by the plastic flow.
After the agitation is sufficiently performed within the workpiece as described above, the first tool is pulled from the workpiece and the softened plate materials are cured. As a result, a plurality of plate materials are joined at points.
Unlike the resistance welding method, in accordance with the friction agitation junction method, plate materials can be joined with a small capacity of power and equipment for supplying a cooling water is not required. Further, as a contamination or a wear does not occur in a junction tool, the tool can be used for a long time without maintenances. Thus, advantages such as a reduction in a running cost and an easy maintenance can be obtained.
Moreover, unlike the mechanical fastening method by rivets, the friction agitation junction method has advantages that an increase in cost caused by rivet materials and an increase in the weight of the resultant joined material do not occur.
In order to improve a junction strength in the friction agitation junction method, the amount of sinking of the first tool into the workpiece must be relatively increased and the range that plastic flow occurs within the workpiece (plastic flow range) must be expanded.
In accordance with the friction agitation junction method, however, the thickness of the workpiece (plate material) is reduced by the amount that the first tool is sunk into the workpiece. Thus, when the amount of sinking of the first tool into the workpiece becomes large, the thickness of the plate material itself is significantly reduced, resulting in a decrease in the strength of the plate material itself. For this reason, there arises a problem in that although a junction strength is improved by sufficiently agitating the workpiece, the strength of the plate material is decreased, so that the strength of the junction portion is decreased.
In accordance with the friction agitation junction method, the distal end (pin) of the first tool must be sunk into the vicinity of the superimposed surface of the workpiece. For this reason, when the thickness of a plate material to be joined is large, the distance between the surface of the workpiece and the superimposed surface is long, so that the amount of sinking of the first tool must be increased. Thus, as described above, the thickness of the plate material is significantly reduced, resulting in a decrease in the strength of the plate material itself. When the thickness of the plate material is large, a radiation capacity becomes large and thus plastic flow hardly occurs within the workpiece. Thus, the workpiece cannot be sufficiently agitated and a junction strength is decreased. In this way, in accordance with the conventional friction agitation junction method, a certain constraint about the thickness of joinable plate materials exists.
The friction agitation junction method is suitable for joining relatively soft plate materials made of, e.g., aluminum alloys. Nevertheless, the tensile characteristics (tensile strength and proof stress) of a 5000 series alloy (Al—Mg) at high temperatures are significantly different from those of a 6000 series alloy (Al—Mg—Si). Thus, when plate materials made of a 6000 series aluminum alloy with relatively low strength are joined, a sufficient junction strength can be obtained. On the other hand, in the case where plate materials made of a 5000 series aluminum alloy with relatively high strength are joined, a sufficient junction strength cannot be obtained because a workpiece is hardly agitated. As described above, a certain constraint about joinable plate materials exists in a conventional friction agitation junction method.